N-containing sugars from Morus alba and their glycosidase inhibitory activities

The reexamination of N-containing sugars from the roots of Morus alba by improved purification procedures led to the isolation of eighteen N-containing sugars, including seven that were isolated from the leaves of Morus bombycis. These N-containing sugars are 1-deoxynojirimycin (1), N-methyl-1-deoxynojirimycin (2), fagomine (3), 3-epi-fagomine (4), 1,4-dideoxy-1,4-imino-D-arabinitol (5), 1,4-dideoxy-1,4-imino-D-ribitol (6), calystegin B2 (1 alpha,2 beta,3 alpha,4 beta-tetrahydroxy-nor-tropane, 7), calystegin C1 (1 alpha,2 beta,3 alpha,4 beta,6 alpha-pentahydroxy-nor-tropane, 8), 1,4-dideoxy-1,4-imino-(2-O-beta-D-glucopyranosyl)-D-arabinitol (9), and nine glycosides of 1. These glycosides consist of 2-O- and 6-O-alpha-D-galactopyranosyl-1-deoxynojirimycins (10 and 11, respectively), 2-O-, 3-O- and 4-O-alpha-D-glucopyranosyl-1-deoxynojirimycins (12, 13, and 14, respectively), and 2-O-, 3-O-, 4-O- and 6-O-beta-D-glucopyranosyl-1-deoxynojirimycins (15, 16, 17, and 18, respectively). Compound 4 is a new member of polyhydroxylated piperidine alkaloids, and the isolation of 6 is the first report of its natural occurrence. It has recently been found that the polyhydroxy-nor-tropane alkaloids possess potent glycosidase inhibitory activities. Calystegin A3 is the trihydroxy-nor-tropane, and calystegins B1 and B2 are the tetrahydroxy-nor-tropane. Calystegin C1, a new member of calystegins, is the first naturally occurring pentahydroxy-nor-tropane alkaloid. The inhibitory activities of these compounds were investigated against rat digestive glycosidases and various commercially available glycosidases.